The invention relates to an active ferrite core radio receiver antenna combined with an inductive and capacitance network for receiving and conveying signal energy between the active antenna and receiver. The active antenna comprises a resonant circuit which is electronically tunable to a frequency of interest.
Ferrite core antennas are widely used as antennas for radio receivers, particularly for AM broadcast band radios, to increase radiation resistance over that simple Hertzian dipole antenna. This is done by forming the conducting element of the antenna in a coil or loop, and placing a ferrite body inside the loop. The ferrite has the effect of concentrating and intensifying the received magnetic field inside the loop. This is the result of the high permeability, xcexc, of the ferrite material. The incorporation of ferrite into the antenna coil is most easily accomplished by winding the antenna coil around the ferrite rod.
The use of a ferrite rod increases the loop""s radiation resistance by a factor of xcexce2, where xcexce, is the ferrite""s effective magnetic permeability. Typically, for frequencies in the 100 to 2000 kHz range, the value of xcexce for typical ferrites is from about 100 to about 10,000. Thus, for a value of xcexce of, e.g., 1,000, ferrite can increase the antenna""s radiation resistance, over a Hertzian dipole, by a factor of 1,000,000.
The ferrite core or rod itself tends to absorb some of the signal power. This represents the work done in xe2x80x9cflippingxe2x80x9d the alignment of the magnetic domains inside the core with each signal element. This ferrite rod xe2x80x9cferrite resistancexe2x80x9d adds resistance in series with the antenna. Even with this added resistance, the antenna""s resistance is typically just a few ohms, or even one ohm or less. However, in operation, the antenna must be coupled to a large impedance of the receiver electronics. This is usually accomplished by adding a capacitor to turn the antenna loop into a resonant circuit.
However, when a signal enters a ferrite rod antenna that is tuned to resonance with a coil and capacitor, the magnetic lines of flux begin to saturate. This is due to sympathetic resonance. Sympathetic resonance is much like setting two guitars close to each other and plucking the string on one of the guitars. If both strings are tuned the same, the plucked string sound waves will cause the corresponding string on the other guitar to vibrate identically. When this saturation occurs in a ferrite core antenna, the ferrite rod antenna takes on polarity, much like a standard magnet. Ferrite antennas of the prior art have only one pick-up coil, and this coil receives the sympathetic resonance from only one-half of the ferrite rod. This results in loss of efficiency and limits the signal-to-noise ratio available to the receiver electronics.
Thus, there is a need for improved efficiency and a higher signal-to-noise ratio antenna that does not saturate at high frequencies, and it is to these ends that the present invention is directed.
The invention provides an antenna for a radio receiver that has high efficiency, high signal-to-noise ratio and that does not saturate at high frequencies. This is accomplished by an antenna structure which employs a ferrite core having two (or more) coils coupled together and located on the ferrite core such that the magnetic fields coupled to the coils induce signals which combine to produce a resulting signal level equivalent to the combined signals in the coils. The coils may be coupled through a transformer where the combined signals of the coils are received in the primary of the transformer. The transformer coupling the antenna coils can dramatically narrow the bandwidth of the received signal. In addition, the antenna coils and the transformer windings may be connected to a capacitor to form a resonant circuit. The capacitor may be variable so the resonant frequency of the antenna may be set by tuning the capacitor. This results in an increased signal level and reduced interference and noise.